To provide an automatic adjusting mechanism for a backrest which never causes a lot of trouble in adjusting a height of a headrest and a backrest and never gives an uncomfortable feeling to a patient.

When a headrest elevating and lowering switch is turned on upward, a headrest elevating and lowering motor 6 fixed on the backrest 3 rotates, and a headrest slider block 12 threadably mounted on a threaded shaft 10 is elevated by rotations of the threaded shaft 10. Consequently, the headrest 2 fixed on the headrest slider block 12 through a headrest slider 13 is elevated. Meanwhile, a backrest elevating and lowering motor 7 rotates by working simultaneously with the operation of the motor 6, and the backrest 3 is elevated together with the motor 7 by the rotations of the threaded shaft 17 threadably mounted on a backrest slider block 20 fixed on a backrest stay 19. At the time, the height of the backrest 3 corresponding to the height of the headrest 2 is automatically adjusted from ergonomic data by recognizing positions of patient's ears.